🔧 Move cargo-show-asm to compile fallback 🔧

Fresh build of the previous commit (318d7ab) confirmed all 4
previously-moved tools (cargo-modules/get/cache/readme) now install
successfully via the compile-fallback loop, but cargo-show-asm was
still silently missing from the image.

Root cause: pacak/cargo-show-asm publishes no GitHub Releases at all
(confirmed via GitHub API: 404 on /releases/latest and empty
/releases list) — the prebuilt-only loop's `|| true` silently skips
it, same failure mode already documented for the other 4 tools.

Also confirmed cargo-c is NOT missing — it installs cargo-cbuild,
cargo-cinstall, cargo-capi (not a binary literally named `cargo-c`),
same false-positive pattern as cargo-edit/cargo-update/etc.

- Dockerfile: removed cargo-show-asm from the prebuilt-only loop,
  added it to the compile-fallback loop and updated the comment
  above that loop to include it in the no-release-assets list

Dockerfile

@@ -146,7 +146,6 @@ RUN --mount=type=cache,id=cargo-registry-native,sharing=shared,target=/usr/local
       cargo-c \
       cargo-binutils \
       cargo-bloat \
-      cargo-show-asm \
       cargo-criterion \
       cargo-careful \
       cargo-public-api \
@@ -175,8 +174,8 @@ RUN --mount=type=cache,id=cargo-registry-native,sharing=shared,target=/usr/local
 # cargo-public-api, cargo-spellcheck, cargo-dist, sea-orm-cli confirmed via
 # `cargo binstall` "Fallback to cargo-install is disabled" on both amd64 and
 # arm64 (a 404 from every prebuilt source, not a rate-limit issue);
-# cargo-modules, cargo-get, cargo-cache, cargo-readme have no GitHub release
+# cargo-modules, cargo-get, cargo-cache, cargo-readme, cargo-show-asm have no
-# assets published at all, same net effect. For just these, allow the
+# GitHub release assets published at all, same net effect. For just these, allow the
 # compile fallback: this stage already cross-links via the
 # zig wrapper scripts above, so compiling here is a native cross-build, not
 # QEMU emulation, and stays fast for this small a tool set.
@@ -199,7 +198,8 @@ RUN --mount=type=cache,id=cargo-registry-native,sharing=shared,target=/usr/local
       cargo-modules \
       cargo-get \
       cargo-cache \
-      cargo-readme; \
+      cargo-readme \
+      cargo-show-asm; \
     do \
       cargo binstall -y --target "${RUST_TARGET}" "${tool}" || true; \
     done